1. Field of the Invention
The present invention generally relates to spinal fixation systems and the like. More particularly, the present invention generally relates to a spinal plate system that includes a mechanism for fixably attaching screws to a plate.
2. Description of the Related Art
The use of spinal fixation plates for correction of spinal deformities and for fusion of vertebrae is well known. Typically, a rigid plate is positioned to span bones or bone segments that need to be immobilized with respect to one another. Bone screws may be used to fasten the plate to the bones. Spinal plating systems are commonly used to correct problems in the lumbar and cervical portions of the spine, and are often installed posterior or anterior to the spine.
Spinal plate fixation to the cervical portion of the spine can be risky because complications during surgery can cause injury to vital organs, such as the brain stem or the spinal cord. When attaching a fixation plate to a bone, bone screws are placed either bi-cortically (i.e., entirely through the vertebrae such that a portion of the screw extends into the spinal cord region) or uni-cortically (i.e., the screw extends into but not through the vertebrae). Uni-cortical positioning of bone screws has grown in popularity because it is generally safer to use. Bi-cortical screws are intended to breach the distal cortex for maximum anchorage into the bone; however, this placement of the screws may place distal soft tissue structures at risk. Screw placement is of particular importance in anterior cervical plate procedures because of the presence of the spinal cord opposite the distal cortex. Unfortunately, because of the soft texture of the bone marrow, uni-cortical screws may undergo movement from their desired positions. In fact, the portion of the bone surrounding such screws may fail to maintain the screws in their proper positions, resulting in screw backout.
Screw backout is particularly a problem when a pair of screws are implanted perpendicular to the plate. When the screws are placed in such a manner, screw backout may occur as a result of bone failure over a region that is the size of the outer diameter of the screw threads. To overcome this problem, a different configuration of the screws has been developed in which two screws are angled in converging or diverging directions within the bone. Advantageously, the amount of bone that is required to fail before screw backout can occur is increased by this configuration as compared to screws which are implanted in parallel. Although positioning screws angled toward or away from each other in a bone reduces the risk of a screw backout, such backouts can still happen. The result of a screw backout can be damaging to internal tissue structures such as the esophagus because a dislocated screw may penetrate the surface of such structures.
In an attempt to reduce the risk of damage to internal tissue structures, some cervical screw plate systems have been devised in which uni-cortical screws are attached to the plate and not just the bone. It is intended that if screw backout occurs, the screw will remain connected to the plate so that it cannot easily contact internal tissue structures. One such system is described in U.S. Pat. No. 5,364,399 to Lowery et al. and is incorporated by reference as if fully set forth herein. This plating system includes a locking screw at each end of the plate which engages the heads of the bone screws to trap them within recesses of the plate. Since the locking screw is positioned over portions of the bone screws, it may extend above the upper surface of the plate. Thus, the locking screw may come into contact with internal tissue structures, such as the esophagus. Unfortunately, breaches to the esophageal wall may permit bacterial contamination of surrounding tissues, including the critical nerves in and around the spinal cord, which can be fatal.
Another plating system that includes a screw to plate locking mechanism is the Aline(trademark) Anterior Cervical Plating System sold by Smith and Nephew Richards Inc. in Memphis, Tenn. A description of this system can be found in the Aline(trademark) Anterior Cervical Plating System Surgical Technique Manual by Foley, K. T. et al., available from Smith and Nephew Richards Inc., September 1996, pp. 1-16 and is incorporated by reference as if fully set forth herein. The bone screws of this system have openings within each bone screw head for receiving a lock screw coaxially therein. Each bone screw may be inserted into a bone such that the head of the screw is positioned within a borehole of a plate placed adjacent to the bone. The head of each bone screw is slotted such that portions of the head may be deflected toward the plate during insertion of the lock screw within the opening of the bone screw. The bone screw may be thusly locked against the plate. Inserting the lock screw into and fixably positioning the lock screw within the opening may be difficult since the lock screw is very small. The surgeon may be unable to hold onto the lock screw without dropping it. Unfortunately, once such a screw falls into the surgical wound, it is typically difficult to retrieve. In some instances it may be unretrievable.
An embodiment of the invention relates to an implant system for fixation of the human spine that includes a plate having end boreholes, midline boreholes, screws, and expandable/contractible rings.
The end boreholes preferably extend from the upper surface to the lower surface of the plate. The end boreholes may be disposed in pairs at opposite ends of the plate. Each end borehole is preferably sized to receive at least a portion of a head of a screw therein. Herein, xe2x80x9cscrewxe2x80x9d is taken to mean any elongated member, threaded or non-threaded which is securable within a bone. Each end borehole is also preferably spherical shaped to permit the screw to be xe2x80x9cobliquely angulatedxe2x80x9d relative to the plate. Herein, xe2x80x9cobliquely angulatedxe2x80x9d is taken to mean that the screw may be positioned at a wide range of angles relative to the plate, wherein the range of angles is preferably from 0 degrees to about 15 degrees from an imaginary axis that is perpendicular to the plate. Since the screws may be obliquely angulated with respect to the plate, the occurrence of screw backout from a bone may be significantly reduced.
The expandable/contractible rings are preferably sized so that they may be positioned within each borehole between the plate and each of the screw heads. The inner surface of each ring is preferably shaped to mate with a screw head while the outer surface is preferably shaped to mate with the plate. The outer surface of each screw head may be tapered such that an upper portion of the head is larger than a lower portion of the head. Each ring may also have a gap that extends vertically through the ring to render it expandable/contractible. Thus, during insertion of a screw head within a bone, the ring preferably exerts a compressive force on the screw head to fixably connect the screw to the plate. The screw may be prevented from contacting tissue structures that are protected by the spine even when screw backout occurs since the screw is attached to the plate.
The midline boreholes may be formed through the plate at various locations along a midline axis extending across the plate. The surface of the plate that surrounds each midline borehole is preferably tapered. Further, the heads of screws that may be positioned within the plates preferably have tapered outer surfaces that are shaped to mate with the tapered surface of the plate. Thus, when such a screw head is inserted into a midline borehole, the shape of the plate causes the screw to become fixably attached to the plate in a position that is substantially perpendicular to the plate. Since the midline boreholes may be used when inserting screws into bone graft, oblique angulation of screws positioned within the midline boreholes is not required.
Prior to surgical implantation of the spinal plate system, the expandable/contractible rings may be placed within the end boreholes of the plate. The plate may then be positioned adjacent to a portion of the spine that requires spinal fixation. Holes may be drilled and tapped into a portion of the bone underlying each end borehole at the desired angle. Screws may be inserted through the end boreholes into the holes, and the heads of the screws may be positioned within the boreholes such that the rings surround at least a portion of the heads. Advantageously, during insertion of the screws, the rings preferably lock the screws in place without occupying regions outside of the boreholes. Further, since the rings are pre-positioned within the end boreholes, surgeons do not have to worry that they may drop the rings during insertion of the screws.
In another embodiment, the head is preferably screwed into the ring to create a fixed connection between bone screw and plate at a selected angle. The screw head preferably contains head threading on its outer surface that is complementary to ring threading contained on the inner surface of the ring. The head threading preferably mates with the ring threading to enhance the connection between the bone screw and the ring. The head preferably has a cavity formed on its upper surface for receiving a driving tool such as a screw driver or an alien wrench.
The outer surface of the head is preferably tapered so that screwing the head into the ring causes a change in width of the ring to fix the bone screw in position relative to the plate. The inner surface of the ring may also be tapered to substantially match the taper on the outer surface of the head. At least a portion of the head preferably has a width greater than the inner width of the ring. As the screw head is screwed into the ring, the ring preferably expands outwardly from its inner surface to accommodate the increasing width of the screw head. The ring may contain one or more slots or gaps as previously described to facilitate expansion of the ring against the inner surface of the borehole.
It is believed that using a threading engagement between the head and ring increases the hoop stress exerted on the head, resulting in a stronger connection between the bone screw and the plate. Moreover, if bone threading becomes loose within a bone, screw backout from plate will tend to be resisted by the threaded connection between the screw head and the ring. Thus, even if the shank loosens within the bone, the head will tend to remain within the borehole of the plate so as not to protrude from the plate into surrounding body tissue.